Lab: Electrolysis of Water

FOR THE TEACHER

Summary Submitted by:
In this lab, students will perform the electrolysis of water Cheryl Ann Park
using a battery, test tubes, thumbtacks, and a plastic cup. Century High School
Santa Ana, California
Grade Level
High school
Thanks to:
Flinn Scientific
AP Chemistry Curriculum Framework
This lab activity supports the following units, topics, and learning objectives:
 Unit 4: Chemical Reactions
o Topic 4.2: Net Ionic Equations
 TRA-1.B: Represent changes in matter with a balanced chemical or
net ionic equation: a. For physical changes. b. For given information
about the identity of the reactants and/or product. c. For ions in a
given chemical reaction.
o Topic 4.7: Types of Reactions
 TRA-2.A: Identify a reaction as acid-base, oxidation-reduction, or
precipitation.
o Topic 4.9: Oxidation-Reduction (Redox) Reactions
 TRA-2.C: Represent a balanced redox reaction equation using half-
reactions.
 Unit 9: Applications of Thermodynamics
o Topic 9.7: Galvanic (Voltaic) and Electrolytic Cells
 ENE-6.A: Explain the relationship between the physical components
of an electrochemical cell and the overall operational principles of the
cell.

Objectives
By the end of this lab, students will
 Understand how electrolysis chemically separates water into hydrogen and oxygen
gas.

Chemistry Topics
This lab supports students’ understanding of
 Electrolysis
 Electrochemistry

Time
Teacher Preparation: 15 minutes
Lesson: 45 minutes

Materials
 9-V battery
 2 metal-only thumbtacks
 Condiment cup

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  2 test tubes
 Baking soda solution
 Phenolphthalein

Safety
 Safety goggles should always be worn when working in a lab.
 Students should wash their hands thoroughly before leaving the lab.
 When students complete the lab, instruct them how to clean up their materials and
dispose of any chemicals.

Teacher Notes
 The baking soda solution doesn’t need to be prepared to any specific
concentration. A small amount of baking soda is needed to allow for the flow of
electricity through water, if distilled water is being used. If using tap water, there
may be enough minerals in your water to omit baking soda. This could be another
extension for this experiment.

FOR THE STUDENT

Lesson
Electrolysis of Water

Background
Electrolysis is the process of breaking molecules into smaller components by using an
electric current. Electrolysis requires an electrolyte solution, so the electrolyte is in the
form of ions. Anions are drawn through the solution to the anode, the positive electrode.
The anode is where oxidation takes place. Oxidation involves the loss of electrons. Contact
with the anode tends to strip off electrons. At the same time, cations move through the
solution to the cathode, the negative electrode. The cathode is where reduction takes
place. Reduction involves the gain of electrons. Contact with the cathode tends to gain
electrons. The process of electrolysis results in new substances.
Electrolysis is used in industry in many ways. By passing an electrical current
through a solution that contains dissolved ions, the ions can participate in chemical
reactions. Depending on the ions, the voltage, and other factors, this can lead to the
formation of new compounds. In electrolysis, electrical energy is used to cause a
nonspontaneous chemical reaction. Electrolysis is often used to obtain elements that are
too chemically reactive to be found free in nature. Electrolysis is used to separate metals
like aluminum, silver, or gold from a compound dissolved in solution. If one of the
electrodes is a metal, it will become covered or plated with any metal in the solution. This
is how objects are electroplated.
Water is a simple chemical made from two gases: hydrogen and oxygen. Every
molecule of water has two atoms of hydrogen for every atom of oxygen. If an electrical
current is passed between electrodes through water, water is split into its two parts in two
reactions: 4 H2O + 4 e- → 2 H2 + 4 OH- and 2 H2O → O2 + 4 H+ + 4 e-
Water has solvent properties because it is polar. Water molecules have charged
ends (+ and -). These charged ends interact with charges on other polar substances to
dissolve them. The presence of ions in solution is what allows water to conduct electricity.
Pure water is, in fact, not a conductor. Water can be made conductive by using an
electrolyte that does not react under the conditions of the experiment.

Purpose
To use electrolysis to separate water into hydrogen and oxygen gas.

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Materials
9-V battery
2 metal-only thumbtacks
Condiment cup
2 test tubes
Baking soda solution
Phenolphthalein

Procedure
1. Label the negative terminal of the battery.
2. Mark two spots on the inside bottom of the condiment cup the same distance apart
as the two terminals of the 9-V battery.
3. Push one tack through one mark so its head is on the outside of the cup, pin poking
inside.
4. Before pushing in the second tack, be sure the two tacks will
not touch.
5. Fill the plastic condiment cup halfway with baking soda
solution.
6. Fill the test tubes completely full with baking soda
solution.
7. One person should hold the cup, and two other people cap the
filled tubes with a thumb and then invert the tubes over the
pins of the thumbtacks. There should be no air in the test
tubes, they should be completely full of solution.
8. Add 2–3 drops of phenolphthalein indicator to the baking soda
solution in the cup.
9. Without allowing the test tubes to tilt at all, place the cup over
the battery so the terminals touch the tacks. Your apparatus should now look like
the diagram. You may want to practice getting the test tubes, thumb tacks, and
battery aligned without baking soda solution first.
10.Observe each test tube, especially the volume of solution. Record your
observations.
11.When one test tube is mostly empty of solution, get your teacher’s attention.
12.Your teacher will light a wood splint. When splint is lit, remove test tube from
negative terminal and cap with thumb.
13.Point the opening of the test tube away from people and place splint into the tube.
Record your observations.
If time permits
Extension A: Repeat steps 12 and 13 to test the gas from the positive terminal but use a
glowing, NOT burning, splint.
Extension B: Repeat the collection of the gases. When one test tube is half filled, turn the
battery, switching the terminals. Collect as much of the mixture of gases as time will
permit. Test the mixtures with a burning splint.
Analysis
1. On the diagram, label the anode and cathode. In each tube,
label the gas formed, the half reaction that occurs, and
whether the half reaction is oxidation or reduction.
2. Explain why pure water does not conduct electricity and will
not undergo electrolysis.
3. Explain why water with baking soda conducts electricity and
undergoes electrolysis.

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 4. What was the purpose of adding phenolphthalein indicator to the solution?
5. How can you tell which test tube is collecting hydrogen gas and which one is
collecting oxygen gas before you test them?
6. Which gas is attracted to the positive electrode and which gas to the negative?
7. Can you explain why each of these gasses prefers a certain electrode?
8. Write a balanced chemical equation for the decomposition of water.
9. Is this reaction endothermic or exothermic?
10.Write a balanced chemical equation for the synthesis of water.
11.Is this an endothermic or exothermic reaction?

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